The present invention relates to electronic component testing, and particularly to a test fixture for holding microstrip hybrid circuits and other microstrip assemblies in such a way that the assemblies can be tested reliably by application of high-frequency signals, with repeatable results.
Test fixtures are currently available which can test small circuit components including microstrip assemblies, but currently available fixtures present certain difficulties in achieving reliable test results. Typical microstrip assemblies include a dielectric substrate in the form of a sheet, with a copper cladding or other conductive material extending over the full area of its bottom face as a ground plane conductor. A much narrower conductive strip, usually with interposed circuit components, is located on the top face of the dielectric sheet to carry the desired signals. Since the geometries of both the microstrip assembly and its input/output connections are critical to its impedance, it is extremely important that test connections to such microstrip assemblies be accomplished precisely at the intended places and essentially identically for each individual microstrip assembly of a series being tested.
Typically, input/output connections to a microstrip assembly to be tested are made by means of launchers, which mate with coaxial cables and are designed to conduct high-frequency signals while maintaining a characteristic impedance through the transition from coaxial cable to microstrip assembly. The surface area presented by the edge of the ground plane conductor of a microstrip assembly is extremely small and makes connection directly between a launcher and the ground plane conductor unreliable, using previously available technology, unless a conductive carrier is provided beneath the microstrip assembly in abutment with the ground plane. Previously available test fixtures therefore require that microstrip assemblies be mounted on such a carrier, which provides for connection with the ground plane conductor while also compensating for any variations in substrate thicknesses. Fastening such microstrip assemblies to carriers, however, requires excessive time and adds to the cost of testing. Likewise, manually positioning probes or other connectors to introduce bias voltages and the like during testing requires more time than desired.
Currently available testing fixtures for microstrip assemblies also suffer from difficulty in achieving reliable repeatability of test results, because of such factors as wear to contact surfaces due to repeated connection and disconnection of the microstrip assemblies, and critical requirements for alignment of the microstrip assemblies relative to the launchers. Thus, previously available test fixtures for microstrip circuit assemblies cannot be relied upon to present the same impedance in electrical connections to several different microstrip test assemblies of identical structure unless extraordinary, time-consuming care is exercised.
A drawback to flexible use of the available test fixtures is that their connectors, for making detachable electrical interconnection with microstrip devices being tested, are arranged coaxially with one another, and are thus difficult to use where contacts on the device to be tested are not located coaxially.
What is needed, then, is a durable, high-production test fixture which is able rapidly to interconnect launchers detachably to microstrip assemblies with predictably reliable and repeatable results. Such a test fixture should be able to test microstrip assemblies without the need for carriers (although they may be used if desired), and to test assemblies of different sizes and including opposite terminal portions which are not coaxial. It is also desired to provide such a test fixture which automatically compensates for different thicknesses of substrates of microstrip assemblies to be tested, which provides for rapid insertion and removal of assemblies to be tested with a minimum of wear on connecting surfaces, and which provides for coordinated and precise electrical interconnection with test assemblies at points other than the terminals of the microstrip conductor.